This may be nitpicking but............DL loaded a new Summer 2013 timetable with some very subtle changes in flight timings and equipage, thus breaking the regularity of its daily flight program. Could anyone clarify why they do this?

They change aircraft usually due to demand and when certain types of aircraft are available on certain days of the week. The time changes are just general estimates as to when they are scheduled and they may need to change due to changes at each airport. Most airlines change and do not let their passengers know if it is just 5, 10 or 15 minute changes, but at least DL has continued the NW policy of coming up with publishing such minute changes, although in the grand scheme of things it really doesn't make a whole lot difference if you arrive 5 or 10 minutes either early or late of their scheduled time. Schedules are not set in stone as they may very due to weather or traffic delays anyway. I have yet to depart and land exactly as they are scheduled.

If you think that's bad, you should take a look at United's schedules.

Their schedules vary from day to day, and week to week. Just check a domestic hub-hub route such as ORD-SFO, and see how many columns/pages it takes to show the entire schedule over the next month. It's almost impossible to figure out how many flights per day they operate because there are so many variations you can't keep track of them all.

Quoting factsonly (Thread starter):This may be nitpicking but............DL loaded a new Summer 2013 timetable with some very subtle changes in flight timings and equipage, thus breaking the regularity of its daily flight program. Could anyone clarify why they do this?

The actual reason this now happens is that a computer optimizer program is creating the schedule. In the old days a person was doing it and he/she didn't go to the trouble of making such minor changes. Now the software is optimizing it to the minute. Departure and arrival times change connection opportunities among other things.

Minor differences from day to day can also be due to problems obtaining consistent daily slot times. Or it could reflect gate availability either at the origin or destination which may vary from day to day when all flights aren't daily.

Many schedules change seasonally due to differences between average summer and winter winds.

Another factor is that flight crews largely get paid based on block times. If there is historical data to suggest a flight is a few minutes shorter on a given day of the week, then the airline will take advantage of that considering crews get paid literally by the minute.